PALEOZOIC BUYOZOA 253 



the Mississippian embayment. The prevailing types in the Ordovician, 

 in which deposits they for the first time attain any considerable abun- 

 dance, belong to the solid massive forms known as the Trepostomata. 

 Beginning with the Silurian, the lacelike Fenestellidse, a large family of 

 the Cryptostomata, become by far the most abundant representatives of 

 the class. The bifoliate Cryptostomata began early in the Ordovician 

 and continued practically to the close of the Paleozoic, while the Cyclo- 

 stomata, which are likewise old, continue to the present. The Treposto- 

 mata, including mainly massive and branching colonies, have been of the 

 greatest service in stratigraphic correlation. This is, first, because of 

 their great abundance and widespread distribution, and, second, because 

 of the certainty and relative ease with which the species can be positively 

 determined by means of thin-sections. Even small imperfect specimens 

 can be determined beyond doubt. The bifoliate Cryptostomata are of 

 nearly equal service, but in these the shape and surface characters are of 

 more importance, requiring greater perfection in preservation to insure 

 positive determination. As to the Fenestellidse and other delicate types, 

 whose discrimination depends solely on easily effaced surface characteris- 

 tics, these are of correspondingly less practical value for stratigraphic 

 purposes. However, at their worst, a long experience among the Paleo- 

 zoic fossils has shown that the Bryozoa compare favorably as guide fossils 

 with any other class of organisms. 



Considered in their paleogeographic bearings, the abundance of the 

 Bryozoa and their occurrence in nearly all kinds of deposits may be said 

 to establish the prevailing shallowness of the continental seas in which 

 they flourished, while a comparative study of the species shows differ- 

 ences in geographic distribution which can be attributed only to localiza- 

 tion of origin and development and dependence on currents for their 

 transportation. In some cases many genera are represented only in 

 faunas which can be traced to invasions of a particular sea. The latter 

 is of especial importance in paleogeography, in that their abrupt geo- 

 graphic limitation suggests considerable detail in the pattern of the con- 

 tinental seas and lands. For example, regarding certain clearly discrimi- 

 nated faunas found in sediments that wedge out northwardly by overlap, 

 we may be certain that they invaded through some opening in the south. 

 Further, we may infer that the origin and dispersal of the fauna lies in 

 one of the permanent oceanic basins in that direction. On the other 

 hand, if beds and faunas extend and terminate in a similar manner in a 

 southward direction, the sea in which the fauna originated and developed 

 is for like reasons located to the north. We then have introduced into the 



